I think Groovy is a wonderful language. However, I would not advise a complete rewrite of your project in Groovy!

We can however use Groovy to test our Java code.

I’m actually a big fan of this approach. I’ve been using it to test legacy applications written in Java.

I still couldn’t use lambdas or fancy Java 8 features but all of a sudden, my test code was more expressive. I could take advantage closures, power asserts, the Spock DSL and Groovy simple syntax. The best part: every library I used was a test dependency and never impacted the actual code.

In this article I will show you how to add Groovy tests to an existing Java application built either with Maven or Gradle.

The code source of a demo application using Maven and Spock is available on github.

Why groovy?

Groovy is a dynamic language with optional typing. It means that you can have the guarantees of a type system when it matters and the versatility of duck typing when you know what your are doing.

Groovy removes all the verbosity from the Java syntax. Some small examples:

// map literals
Map<String, String> things = ['hello': 'world']

// Write to a file
new File("hello.txt") << 'Hello world!'

// Add some numbers
BigInteger a = 18
BigDecimal b = 24
int sum = a + b
println "$sum ${sum.class}" // 42 class java.lang.Integer

// List literals
List<Number> numbers = [-2, 12, 6, 3]
// Closures
def result = numbers
        .findAll { it > 0 } // filter
        .collect { it * 2 } // map
        .sum() // reduce

// template strings
println "This answer to life, universe and everything: ${result}"

If you want a good introduction to groovy check out the groovy style guide.

You can also watch the amazing Groovy for Java developers presentation by Peter Ledbrook.

Another thing. Groovy let you access private class members. Although this completely violates encapsulation, you will get away with just a warning.

It is nice to have this kind of ability when you add tests to a legacy application before refactoring it.

Why Spock?

Spock is a wonderful test framework.

It combines the best features of other frameworks like JUnit, jMock, and RSpec and let you write specifications with a nice BDD DSL.

It is fully compatible with JUnit so you can use all the stuff you like (rules for instance) and much more!

It will also completely remove the need for a mocking framework like Mockito.

If you want to learn more about Spock read: why spock and spock primer.

I also found the next level spock repo interesting to look at.


You are now ready to add Spock to your tool-belt. But how?

With Maven

Add dependencies to Groovy and Spock:


Now, you need to tell maven to compile the code contained in src/test/groovy. We will use the gmavenplus plugin for that.

        <!-- Optional -->

By convention, Spock tests are called specifications and spec files end with *Spec.groovy. This is optional but we can enable that behavior by overriding the surefire default configuration.

With gradle

Just apply the groovy plugin, included by default in Gradle.

apply plugin: 'groovy'

Since the plugin extends the Java convention, it will automatically compile the Java code contained in src/main/java and src/test/java as well as the Groovy code contained in src/main/groovy and src/test/groovy.

Your first Spock specification

Place this little specification in src/test/groovy:

import spock.lang.Specification
import spock.lang.Unroll

class MySpec extends Specification {

    def "max(#a,#b) == #c"() {
        // This class is in our Java code
        MyClass.max(a, b) == c

        a  | b   | c
        1  | 2   | 2
        42 | -12 | 42
        42 | -12 | -42

Here is what it looks like in IntelliJ:

You can also verify that it works with maven by typing:

mvn test

You would get this result:

 T E S T S
Running MySpec
Tests run: 3, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.146 sec <<< FAILURE! - in MySpec
max(42,-12) == -42(MySpec)  Time elapsed: 0.105 sec  <<< FAILURE!
org.spockframework.runtime.SpockComparisonFailure: Condition not satisfied:

Math.max(a, b) == c
     |   |  |  |  |
     42  42 -12|  -42

        at MySpec.max(#a,#b) == #c(MySpec.groovy:9)

Results :

Failed tests:
  MySpec.max(#a,#b) == #c:9 Condition not satisfied:

Math.max(a, b) == c
     |   |  |  |  |
     42  42 -12|  -42

Tests run: 3, Failures: 1, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.887 s
[INFO] Finished at: 2016-01-20T15:52:40+01:00
[INFO] Final Memory: 11M/309M
[INFO] ------------------------------------------------------------------------

IDE integration

Install the gmavnen intelliJ plugin and the spock plugin for a better integration with your IDE.


Even if your whole codebase is in Java, your fellow developers will thank you for bringing a breath of fresh air to your project with Groovy tests.

The configuration to get this working is very simple. You have no excuse for not giving it a try!

As always, checkout the github repo and tell me your thoughts.

Geoffroy Warin

Hello! I write codez

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